Tomato is the natural successor, replaces the Linksys GUI with many new features, but similar code base under the bonnet - the original Tomato author was also responsible for HyperWRT Tofu, which was merged into Thibor's version IIRC.

Keep an eye on www.tomatousb.org, IPV6 with Linux 2.6 kernel is in active development in git, likely to make next beta release. Whether your old router is up to it is another matter - wrt54gs v1.1 and 2 had 32MB ram, but have old wireless chips, not too clear if supported by the latest ND drivers.